As described in US Patent Application No. 2011/0090604, it is desirable to protect an AC load from excessive power through the use digitally controlled bidirectional switching. In such topologies, the AC input voltage ACIN, optional feedback signals FBAK, and optional current monitoring signals IMON are continuously monitored by a system controller that is operable to process the monitored signals and look for overage conditions. Upon such overage conditions, the system controller is typically implemented to generate one or more Pulse Width Modulated PWM signals proportionate to the amount of desired AC power reduction. The said PWM signals are then transmitted to one or more bidirectional switches arranged in a configuration that provide series AC switching and parallel AC switching. Typically the bidirectional switches connected in series with the AC input ACIN and the load are implemented with a bypass path that allows AC current flow prior to detecting the AC overage condition, wherein the said bypass path is many times implemented with a normally closed electromechanical relay, or a normally conductive MOSFET device such as a depletion MOSFET. Unfortunately, depletion MOSFET devices are relatively high priced and generally require higher power enhancement mode devices in parallel to provide a less expensive conductive path for the AC current flow. Electromechanical relays do provide a less expensive solution versus depletion MOSFETS, but degrade the product lifecycle and exhibit a relatively slow response time when actuating, 3 to 5 msec, wherein a sensitive AC load may be damaged as a result of slow response time.
Considering the AC voltage monitoring techniques, typically the AC input voltage ACIN or AC output voltage ACOUT are directly connected and monitored by the system control stage. In these topologies the system control stage may not be electrically floating with respect to the AC input signal ACIN or the AC output signals being fed back FBAK. As a result of not providing electrical isolation for the system control stage, outside communication with the system control stage, such as USB or other digital communication schemes, typically require high-speed digital isolators and using one system control stage to monitor and control multiple channels of AC bidirectional switching becomes problematic. In other words, multi-channel systems with a plurality of AC inputs ACINS would typically require multiple system controllers, wherein each AC input signal ACIN would be monitored by its own system control stage. Such a topology would not be ideal for multi-channel AC attenuation and protection devices due to increased system cost and complicated communication schemes between adjacent AC channels. Additionally, in systems utilizing current monitoring for determination of load power or load impedance, math intensive computations are needed which typically require a system controller with suitable multiplication capability.
Considering the powering options, applications have generally incorporated a high efficiency power supply operable to derive necessary power for all operational stages, including the system control stage, signal conditioning stage, isolation stage, and bidirectional switching stages, from the inbound AC input signal ACIN. In the past, this ACIN power supply stage has been required to turn on or enable well before the detection of overages. As mentioned earlier, designs incorporating depletion MOSFET devices for use as an AC bypass path have required the power supply to turn on prior to an overage condition in order to activate the gate drive on higher power enhancement mode MOSFET devices connected in parallel with the depletion devices. As a result, the enhancement mode MOSFET devices enable prior to the detection of overage conditions, and due to their significantly reduced ON resistance, the enhancement mode MOSFETs relieve the depletion MOSFET devices from any significant electrical current flow. This topology has been demonstrated to work very well; however, enabling the power supply that is fed from the AC input signal ACIN can result in measurable harmonic distortion on the inbound AC line, thereby increasing the harmonic distortion at the load.